The present invention generally relates to brakes and more particularly to a fail-safe braking apparatus for a load driven and controlled by a linear motor. When electrical energy is removed from the linear motor that drives the load, the braking apparatus is actuated to prevent the load from responding to the force of gravity or coasting.
Rail brake devices for linear motor driven loads are well-known. Generally, these types of brakes employ brake pad carrying caliper arms that apply opposed clamping forces to both sides of a linear braking rail upon being activated by a spring or an electromagnetic device. Such an apparatus is shown in U.S. Pat. No. 5,518,087 to Sang Y. Hwang for Rail Brake Apparatus for a Linear Motor Elevator. 
Another prior art type of brake for a linear motor driven load is disclosed in U.S. Pat. No. 5,950,773 to Osamu Ito for Braking Apparatus For A Linear Motion Guiding Apparatus. Ito discloses a pair of mutually opposed braking pads 58 and 59 that clamp onto both sides of a steel belt 63 in order to provide braking for the load. A pair of linkages moves the braking pads into and out of engagement with the steel belt. In normal operation, a piezoelectric actuator is energized with an electric voltage to cause bending of a linkage. The distortion of the linkage removes braking pressure from the clamping brake pads. Loss of electrical voltage removes the elongating force provided by the piezoelectric actuator, allowing the restorative force of the bent linkage to move the brake pads into clamping engagement with the steel belt.
In the aforementioned patented apparatus, as well as in other forms of linear motor braking devices, the braking force is applied through an external means such as the restorative force of the distorted linkages of Ito or the electromagnetic force of the device shown in the Hwang patent. Other conventional means of preventing a vertically oriented linear motor load from falling include pulley/cable and counterweight systems and pneumatic balancing devices. In all such apparatus the applied braking forces must be commensurate with the mass of the load to which the braking action must be applied.
It is, therefore, the primary object of the present invention to provide a fail-safe brake for a linear motor driven load that will be activated upon failure of the electrical power to the linear motor.
A second and significant object of the present invention is to provide a self-actuating brake that does not require externally applied forces to implement the braking function.
A third object of the invention is to provide a self-actuating brake that minimizes any required spring force and maximizes the braking force, thereby keeping the brake small, relative to load size, simple and easy to maintain or replace.
Another object of the invention is to provide a braking apparatus for a linear electric motor driven load that will stop any further movement of the load, either by virtue of the load""s coasting or as a result of gravitational pull, once electric power to the linear motor has failed.
A still further object of the invention is to provide a brake for a linear motor that requires only a very minimum of electrically activated force to restrain the braking apparatus from braking action during periods when electrical power is available and the load is under the motor""s control.
Another object of the invention is to provide a brake for a linear motor that, by slight variations in the design of the geometry of the brake and/or the materials of the respective braking surfaces, can be made to provide a quick sudden stop of the load or a slow soft stop.
These and other objects, features and advantages of the present invention will become apparent upon a reading of the following summary and detailed description of the invention.
The braking apparatus of the present invention is functional to stop the gravitationally motivated movement or the coasting motion of a load carrying member that is movable on the bed of a linear motor. The load is moved and controlled by the linear motor, the armature coils and field magnet for which are respectively coupled to either the load carrying member or the bed. When electrical power to the linear motor is removed or fails it is important that the load be stopped and not allowed to free fall or coast, with respect to the bed. This is especially true when the load carrying member moves in a vertical or angular direction, but also when the load travels horizontally.
Accordingly, the present invention provides a self-actuating brake for the load carrying member that functions immediately upon power failure to the linear motor to stop the load carrying member. The geometry of the braking arm and the principle of its operation allow the size of the brake to be disproportionately small compared to the mass of the load.
Similar in its basic operation to that of a pivotally mounted door stop, the load brake of the present invention comprises a brake arm having a brake pad at one of its ends and at its other end a pivotal attachment to the linear motor""s load, or load carrying member. To urge the brake pad into initial engagement with a braking surface that is integral with the motor""s bed, the brake arm is lightly spring loaded. In normal operation, when electric power is being supplied to the linear motor and the load is under control, the brake pad is held out of engagement with the braking surface by an electrically operated solenoid. The solenoid is operated from the same electrical source that supplies energy to the linear motor. When electrical power fails, or is removed from the linear motor, the solenoid is also deenergized, releasing the brake arm and allowing spring biasing pressure to bring the brake pad into engagement with the bed""s braking surface.
Once the brake pad makes contact with the braking surface, the brake pad is frictionally pulled, by the movement of the load, into greater and greater binding engagement with the braking surface. The rate of load deceleration, that is, whether the stop is substantially sudden or whether it is a xe2x80x9csoftxe2x80x9d stop is determined by the geometry of the brake arm and the material chosen for the brake pad. The elements of geometry that effect deceleration include the angle of the brake arm with respect to the braking surface and the relative location of the brake arm pivot point. The type of material used for the brake pad and its hardness also effects the rate of deceleration.
A brake arm and brake pad, in accordance with the present invention, eliminate the need for large external forces to apply braking pressure against the bed""s braking surface or for large solenoids to restrain the brake arm against large spring braking forces during normal operation. The inventive construction allows the braking mechanism to be compact and inexpensive. Because the brake works against only one side of the bed""s braking surface, removal and replacement of the brake apparatus is facilitated.